The present invention is generally directed to processes for the preparation of homopolymers and copolymers. More specifically, the present invention relates to improved polymerization processes which provide homopolymer and copolymer resin products which possess narrow polydispersity properties and which polymerization processes proceed with high monomer to polymer conversion. In particular, this invention relates to improved stable free radical mediated or pseudoliving polymerization processes which yield homopolymers and copolymers having number average molecular weights (M.sub.n) above about 100 to about 1,000 and having a polydispersity ratio of the weight average molecular weight (M.sub.w) to the number average molecular weight (M.sub.n) of from about 1.0 to about 2.0 and which processes accomplished in accordance with the present invention provide numerous operational and economic advantages associated therewith.
The present invention provides in embodiments a pseudoliving polymerization process that enables the synthesis of narrow polydispersity homopolymer and copolymer resins from a variety of free radical reactive monomers. The process can, in embodiments, use known free radical initiators in combination with specific non-nitroxide type stable free radical agent compounds and free radical reactive monomers to afford narrow polydispersity thermoplastic resins or elastomers.
The free radical initiator compounds selected must satisfy certain maximum half-life requirements for the purpose of providing efficient and highly controlled polymer chain initiation. The stable free radical agent compounds selected must satisfy certain stability requirements including high thermal stability, and high resistance to degradation by acids or photochemical means. The non-nitroxyl type stable free radical compounds must not initiate nor excessively inhibit or retard desired polymerization processes. The stable free radical agent compounds selected must also satisfy certain bond energy and steric hindrance requirements when the stable free radical agent compound is attached to the end of the propagating polymer chain.
In other embodiments the polymerization processes of the present invention can be used to prepare block copolymers and multi-block polymer having narrow polydispersity properties wherein at least one of the blocks is optionally water soluble thereby providing, for example, a means for preparing surface active or surfactant materials having well defined polydispersity and hydrophobe-lipophobe balance (HLB) properties.
Many polymerization processes used for the synthesis of narrow polydispersity resins, such as anionic, cationic, and group transfer polymerization processes, are severely limited by the need for anhydrous reaction conditions and monomers which do not contain protic or reactive functional groups, for example, hydroxy (OH) carboxy (CO.sub.2 H), amino (NH), and the like. As a consequence, these processes are not readily applicable to the polymerization of functionalized monomers since these monomer materials tend to be hydroscopic and any associated water may readily destroy the polymerization initiator component, for example, the hydrolysis or protonation of organolithium reagents by the monomer in anionic polymerization processes, or in other ways cause the polymerization to fail entirely or to be industrially inefficient.
It is generally accepted that known anionic and cationic polymerization processes used for the preparation of narrow polydispersity resins, block and multiblock polymers are not believed possible in aqueous or protic solvent containing polymerization media, or the aforementioned protonic or reactive functional groups, reference the aforementioned U.S. Pat. No. 5,312,704.
Of the known polymerization processes a preferred way to prepare polymers or copolymers having a narrow molecular weight distribution or polydispersity is by anionic processes. The use and availability of resins having narrow polydispersities in industrial applications is limited because anionic polymerization processes must be performed in the absence of atmospheric oxygen and moisture, require difficult to handle and hazardous initiator reagents, and consequently such polymerization processes are generally limited to small batch reactors. In addition, the monomers and solvents that are used must be of high purity and anhydrous thereby rendering the anionic process more expensive than alternatives which do not have these requirements. Thus, anionic polymerization processes are difficult and costly. It is desirable to have free radical polymerization process that provides narrow molecular weight distribution homoacrylate containing resins that overcomes the shortcomings and disadvantages of the aforementioned anionic polymerization processes.
Similarly, group transfer polymerization (GTP) processes have limitations and disadvantages, such as anhydrous reaction conditions and expensive reagents, which disadvantage GTP processes particularly for large scale industrial applications.
Free radical polymerization processes are generally chemically less sensitive than anionic processes to impurities in the monomers or solvents typically used and are substantially or completely insensitive to water. There has been a long felt need for an economical free radical polymerization process which is suitable for preparing narrow polydispersity resins in the presence of water.
Conventional free radical polymerization processes that are used to polymerize monomers in general, and functionalized monomers in particular inherently give broad polydispersity resin products or require that sophisticated processing conditions and materials handling protocols be employed.
The present invention, in embodiments, enables the preparation of homopolymers, random copolymers, block and multiblock copolymers, and polymer blends, with operator selectable molecular weight and polydispersity properties, and which preparation was heretofore not achievable in stable free radical moderated, free radical initiated polymerization systems.
The polymer resins produced by processes of the present invention, in embodiments, can be, if desired, essentially monomodal, that is the molecular weight distribution is narrow and indicative of a Poisson character and without shoulders or side bands. In embodiments, by repeating the heating step, comprising the combined initiation and polymerization step, there is provided a means for obtaining monomodal mixtures of polymer resins that are compositionally the same resin type having characteristics of both narrow polydispersity and known or selectable modality greater than 1.
In embodiments, processes of the present invention provide a means for conducting pseudoliving free radical polymerization processes on multi kilogram or larger scales. The aforementioned embodiments may be accomplished in a one or single pot reactor environment. In embodiments, polymeric chain growth proceeds by a pseudoliving mechanism and can provide resins of variable molecular weights from very low to very high, for example, less than about 2,000 up to about 200,000 while maintaining narrow molecular weight distributions or polydispersities. In embodiments, block and multiblock copolymers can be synthesized by the aforementioned stable free radical moderated polymerization processes wherein each block formed is well defined in length by the sequentially added and reacted monomer and wherein each additional block that is formed also possesses a narrow molecular weight distribution.
Polymerization processes known in the art proceed by a free radical mechanism providing resins of broad polydispersities and generally high molecular weights. The present invention relates to polymerization processes that proceeds via a pseudoliving free radical mechanism and provide resins of high, intermediate, or low molecular weights and with narrow polydispersities. The present invention provides polymer products with a latent thermally reactive or latent functional group on at least one end of the polymer which can be used for further reaction to prepare other resins with complex architectures. The present invention, in embodiments, provides polymerization processes that enable control of resin molecular weight, weight distribution, modality of the products, and the like properties.
The present invention is directed in embodiments to pseudoliving polymerization processes which permit the economic preparation of narrow polydispersity resins with low, intermediate, or high molecular weights. The low molecular weight resins can be prepared without a chain transfer agent or molecular weight modifier by, for example, selecting appropriate ratios of reactants, which provides several advantages over conventional chain transfer mediated polymerization processes.
Copolymers prepared by conventional free radical polymerization processes inherently have broad molecular weight distributions or polydispersities, generally greater than about four. One reason is that most free radical initiators selected have half lives that are relatively long, from several minutes to many hours, and thus the polymeric chains are not all initiated at the same time and which initiators provide growing chains of various lengths at any time during the polymerization process. Another reason is that the propagating chains in a free radical process can react with each other in processes known as coupling and disproportionation, both of which are chain terminating and polydispersity broadening reaction processes. In doing so, chains of varying lengths are terminated at different times during the reaction process which results in resins comprised of polymeric chains which vary widely in length from very small to very large and thus have broad polydispersities. If a free radical polymerization process is to be enabled for producing narrow molecular weight distributions, then all polymer chains must be initiated at about the same time and premature termination by coupling or disproportionation processes must be avoided or eliminated.
Practitioners in the art have long sought an inexpensive, efficient and environmentally efficacious means for producing polymers having operator controllable or selectable molecular weight properties, and further, processes which selectively afford a wide variety of different polymer product types and have narrow molecular weight distribution properties.
In the aforementioned U.S. Pat. No. 5,322,912 there is disclosed free radical polymerization processes for the preparation of a thermoplastic resin or resins comprising: heating from about 100.degree. to about 160.degree. C. a mixture comprised of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form the thermoplastic resin or resins with a high monomer to polymer conversion and a narrow polydispersity. A broad spectrum of free radical reactive monomers are suitable for use in the highly versatile polymerization process. While the use of acidic promoter compounds disclosed therein enabled the stable free radical mediated polymerization reactions to proceed at industrially acceptable rates, for example, at temperatures of from about 100.degree. to about 160.degree. C., the use of acidic promoter compounds in conjunction with nitroxide or nitroxyl type stable free radical compounds was compromised by one or more competing acid promoted side reactions, for example, degradation of the nitroxyl stable free radical compound, with the result that the costly nitroxide stable free radical compound was being unnecessarily consumed and was being diverted and precluded from an exclusively productive role in the polymerization process, and wherein the exclusively productive role is that of a stable free radical moderating agent and as a thermally labile end group or blocking group in pseudoliving polymerization processes.
The following patents are of interest to the background of the present invention, the disclosures of which are incorporated by reference herein in their entirety:
In European Patent Publication 349,270 B1, filed Jun. 6, 1988 (US), is disclosed a pressure-sensitive adhesive composition characterized by comprising: a block copolymer represented by the general formula I(BAT).sub.n wherein I represents the free radical initiator portion of an iniferter to the formula I(T).sub.n ; T represents the termination portion of said iniferter; n is an integer of at least 2; and B represents an elastic acrylic polymer block having a glass transition temperature of at least 30.degree. C. wherein said A-block is formed of a monomer selected from the group consisting of methyl methacrylate, polystyrylethyl methacrylate, macromer, methyl methacrylate macromer, acrylic acid, acrylonitrile, isobornyl methacrylate, N-vinyl pyrrolidone, and mixtures thereof, the weight ratio of said B-block to said A-block in said block copolymer being from 95:5 to 50:50; and 0 to 150 parts by weight tackifier per 100 parts block copolymer. Also disclosed is a method of making the pressure sensitive adhesive which relies upon mixing and exposing an iniferter of the formula I(T).sub.n to an energy source in the presence of a sequence of monomer charges.
U.S. Pat. No. 4,581,429 to Solomon et al., issued Apr. 8, 1986, discloses a free radical polymerization process which controls the growth of polymer chains to produce short chain or oligomeric homopolymers and copolymers including block and graft copolymers. The process employs an initiator having the formula (in part) .dbd.N--O--X, where X is a free radical species capable of polymerizing unsaturated monomers. The molecular weights of the polymer products obtained are generally from about 2,500 to 7,000 having polydispersities generally of about 1.4 to 1.8, at low monomer to polymer conversion. The reactions typically have low conversion rates and use relatively low reaction temperatures of less than about 100 degrees C., and use multiple stages.
European Patent Publication No. 0135280 corresponding to European Patent Application No. EP 84 304,756 is the European Patent Office equivalent of the aforementioned U.S. Pat. No. 4,581,429.
In Polymer Preprints, 35 (1), 778 (1994), Matyjaszewski et al., is disclosed thermal polymerizations of styrene monomers in the presence of stable radicals and inhibitors, but without a free radical initiator present, such as peroxide or azo compounds.
In U.S. Pat. No. 5,268,437, to Holy, issued Dec. 7, 1993, discloses a high temperature aqueous processes for the polymerization of monoethylenically unsaturated carboxylic monomer to produce low molecular weight, water-soluble polymer products useful as detergent additives, scale inhibitors, dispersants and crystal growth modifies. Suitable monomers include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, crotonic acid, and itaconic acid. The reactions are run at temperatures ranging from about 130.degree. to 240.degree. C., preferably from about 140.degree. to about 230.degree. C., with polydispersities less than 2.5. The process can be continuous, semicontinuous, or batch.
In U.S. Pat. No. 4,546,160, to Brand et al., issued Oct. 8, 1985, is disclosed a process to continuously bulk polymerize acrylic monomers to prepare low molecular weight, uniform polymers employing minor amounts of initiator and, optionally solvents, at short residence times and moderate reaction temperatures to provide high yields of a product with polydispersities less than 3, suitable for high solids applications.
U.S. Pat. No. 5,059,657 to Druliner et al., issued Oct. 22, 1991, discloses a polymerization process for acrylic and maleimide monomers by contacting the monomers with a diazotate, cyanate or hyponitrite, and N-chlorosuccinimide, N-bromosuccinimide or a diazonium salt. The polymer produced can initiate further polymerization, including use in block copolymer formation.
U.S. Pat. No. 4,736,004 to Scherer, Jr. et al., issued Apr. 5, 1988, discloses novel persistent perfluorinated free radicals which, upon thermal decomposition, yield free radical species which can be used to polymerize polymerizable monomers containing ethylenic unsaturation.
U.S. Pat. No. 3,600,169 to Lawton, issued Aug. 17, 1971, discloses an electrostatic light sensitive reproduction sheet employing a composition comprising in an insulating resinous binder an organic stable free radical and a precursor sensitive to light to be converted to transient free radicals reactive with the stable free radical to change the conductance of the sheet so that an electrostatic image can be formed. Also disclosed is an extensive listing of stable free radical compounds.
Other references cited in an international search report for the aforementioned commonly owned U.S. Pat. No. 5,322,912 are: J. Am. Chem. Soc., 1983, 5706-5708; Macromol., 1987, 1473-1488; Macromol., 1991, 6572-6577; U.S. Pat. No. 4,628,019 to Suematsu et al., issued Aug. 10, 1986; U.S. Pat. No. 3,947,078 to Crystal, issued Aug. 10, 1976; and U.S. Pat. No. 3,965,021 to Clemens et al., issued Jun. 22, 1976. The cited references disclose alternative means, as discussed above such as anionic, group transfer, and the like, for preparing polymer resins with narrow polydispersity properties, but which alternative means do not provide the convenience and economic improvements of the present invention.
One known method of achieving control of polymer molecular weight is through the use of efficient chain transfer agents, but this approach has several drawbacks. This approach irreversibly incorporates the structure of the chain transfer agent into the polymer chain. This can be undesirable since that structure will have an increasing effect on the properties of the polymer as molecular weight decreases. Furthermore, the chain transfer agents commonly employed are mercaptans. These materials are expensive and have objectionable odors associated with their presence. Other common chain transfer agents are hypophosphites, bisulfites, halogenated hydrocarbons such as carbon tetrabromide, and alcohols. These also add to the cost of the process, introduce undesired functionally or properties to the polymer, can introduce salts into the product, and may necessitate an additional product separation step to remove the chain transfer agent from the reaction mixture.
Another way of lowering the molecular weight of the polymer product is by increasing the amount of free radical initiator. This approach adds considerably to the cost of production and may result in polymer chain degradation, crosslinking, and high levels of unreacted initiator remaining in the product. In addition, high levels of initiator may also result in high levels of salt by-products in the polymer mixture which is known to be detrimental to polymer performance in many applications. The same is true for chain stopping agents such as sodium metabisulfite.
High levels of metal ions together with high levels of free radical initiator have also been tried as means for controlling molecular weight. This method is taught in U.S. Pat. No. 4,314,044 where the ratio of initiator to metal ion is from about 10:1 to about 150:1 and the initiator is present from about 0.5 to about 35 percent based on the total weight of the monomers. Such an approach is unsuitable for some products, such as water treatment polymers, which can not tolerate metal ion contaminants in the polymer product. In addition, the product is usually discolored due to the presence of the metal ions. Thus, polymerization processes which produce product polymers containing residual non-polymeric materials may be significantly negatively compromised with respect to, for example, appearance, performance and toxicity properties. Polymeric processes which create or contain non-polymeric residual materials are preferably avoided.
The following references are also of interest: U.S. Pat. Nos. 3,682,875; 3,879,360; 3,954,722; 4,201,848; 4,542,182; 4,581,429; 4,777,230; 5,059,657; 5,173,551; 5,191,008; 5,191,009; 5,194,496; 5,216,096; and 5,247,024.
In polymerization reaction processes of the prior art, various significant problems exist, for example difficulties in predicting or controlling both the polydispersity and modality of the polymers produced. These polymerization processes produce polymers with high weight average molecular weights (M.sub.w) and low number average molecular weights (M.sub.n) resulting in broad polydispersities or broad molecular weight distribution (M.sub.w /M.sub.n) and in some instances low conversion. Further, polymerization processes of the prior art, in particular free radical processes, are prone to generating excessive quantities of heat since the polymerization reaction is exothermic. As the viscosity of the reaction medium increases dissipation of heat becomes more difficult. This is referred to as the Trommsdorff effect as discussed and illustrated in Principles of Polymerization, G. Odian, 2nd Ed., Wiley-lnterscience, N.Y., 1981, page 272, the disclosure of which is entirely incorporated herein by reference. This is particularly the situation for reactions with high concentrations of soluble monomer, for example greater than 30 to 50 percent by weight soluble monomer, which are conducted in large scale reactors with limited surface area and limited heat dissipation capacity. Moreover, the exothermic nature of free radical polymerization processes is often a limitation that severely restricts the concentration of reactants or the reactor size upon scale up.
Further, gel body formation in conventional free radical polymerization processes may result in a broad molecular weight distributions and/or difficulties encountered during filtering, drying and manipulating the product resin, particularly for highly concentrated reactions.
These and other disadvantages are avoided, or minimized with the homopolymeric and copolymeric polymerization processes of the present invention.
Thus, there remains a need for polymerization processes for the preparation of narrow polydispersity polymeric resins by economical and scalable free radical polymerization techniques and which polymers retain many or all of their desirable physical properties, for example, hardness, low gel content, processibility, clarity, high gloss durability, and the like, while avoiding the problems of gel formation, exotherms, volume limited and multi-stage reaction systems, complex purification, encumbered or comprised performance properties due to undesired residuals or broad polydispersity properties of the polymer resin products, and the like, associated with prior art free radical polymerization methodologies.
There also remains a need for improved stable free radical moderated polymerization processes wherein specifically selected stable free radical agent compounds are employed for controlling polymerization processes, for imparting greater stability to the polymer products and to the reaction process, and which improved stable free radical agent compounds and polymerization processes overcome the aforementioned limitations and problems.
The polymerization processes and the resultant thermoplastic resin and elastomer products of the present invention are useful in many applications, for example, as a variety of specialty applications including toner and liquid immersion development ink resins or ink additives used for electrophotographic imaging processes, or where monomodal or mixtures of monomodal narrow molecular weight resins or block copolymers with narrow molecular weight distribution within each block component are suitable, for example, in thermoplastic films, electrophotographic marking materials such as toners and toner additives, and aqueous or organic solvent borne coating technologies.